John Schmidt 9ee67b967b stage2: avoid inferred struct in os_version_check.zig
Before this commit, compiling an empty main with Stage 2 on macOS x86_64 results in

```
../stage2/bin/zig build-exe -ODebug -fLLVM empty_main.zig
error: sub-compilation of compiler_rt failed
    [...]/zig/stage2/lib/zig/std/special/compiler_rt/os_version_check.zig:26:10: error: TODO: Sema.zirStructInit for runtime-known struct values
```

By assigning the value to a variable we can sidestep the issue for now.
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This compiler-rt library is ported from LLVM.

It's needed because LLVM emits library calls to compiler-rt when hardware lacks functionality, for example, 64-bit integer multiplication on 32-bit x86.

This library is automatically built as-needed for the compilation target and then statically linked and therefore is a transparent dependency for the programmer.

Any bugs should be solved by trying to duplicate the bug upstream.

  • If the bug exists upstream, get it fixed with the LLVM team and then port the fix downstream to Zig.
  • If the bug only exists in Zig, something went wrong porting the code, and you can run the C code and Zig code side by side in a debugger to figure out what's happening differently.